Hi Ken, Here's the source for one algorithm used in GTEM radiated emission testing. There may be more but this is what I found.
*ETS-Lindgren GTEM Manual:* http://www.ets-lindgren.com/sites/etsauthor/ProductsManuals/TEM_Devices/399250%20REV%20j_MANUAL_GTEM.pdf See page 38 "Radiated Emission Testing - General" for an overview, which describes a 3 position correlation algorithm derived by Wilson, et al. at ABB. Here's one paper that provides the algorithm details: P. Wilson, "On Correlating TEM Cell and OATS Emission Measurements", IEEE Transactions on Electromagnetic Compatibility, Vol 37, No. 1, Feb 1995. Couldn't find this paper as a freebie online, but it's available for download on the IEEExplore.org website (free for IEEE EMC members). After reviewing this algorithm I see it is not a simple transducer function like it is for immunity, but instead it involves significant math computations thereby requiring a computer algorithm. One interesting note is that on page 40 of this same ETS-Lindgren manual it states an MS-DOS utility program is available for download on their website to assist with the calculations (but I've been unable to locate it). *NPL/York Measurement Good Practice Guide 65:* https://www.npl.co.uk/special-pages/guides/gpg65_gtem This link was previously provided on this listserv thread in an earlier e-mail. See page 16-17 "GTEM to OATS Correlation", which references the following paper for the theoretical details: Angela Nothofer, "Cross-polar coupling in GTEM cells used for radiated emission measurements", PhD thesis, University of York, Department of Electronics, December 2000. Here's a link to this thesis but it requires login credentials to download: http://yorsearch.york.ac.uk/primo_library/libweb/action/display.do?tabs=requestTab&ct=display&fn=search&doc=44YORK_ALMA_DS21221784140001381&indx=1&recIds=44YORK_ALMA_DS21221784140001381&recIdxs=0&elementId=0&renderMode=poppedOut&displayMode=full&frbrVersion=&frbg=&&vl(138704577UI0)=any&dscnt=0&scp.scps=scope%3A%2844YORK_ALMA_DS%29%2Cscope%3A%2844YORK_WREO_YORK%29%2Cscope%3A%2844YORK_ATOM_DS%29%2Cscope%3A%2844YORK_DIGLIB_DS%29&tb=t&vid=44YORK&mode=Basic&srt=rank&tab=tab1&dum=true&vl(freeText0)=angela%20nothofer&dstmp=1565825463456 Lastly, here's another downloadable paper by this same thesis author, which on page 4 references the same P. Wilson paper addressed above (plus also references IEC 61000-4-20): https://pdfs.semanticscholar.org/2e32/99409062dc3c683db97281d09891d7419518.pdf In summary, the desired transfer function is not a simple equation but instead is a math intensive computer algorithm. Hope this is helpful. Manny Barron [email protected] On Tue, Aug 13, 2019 at 11:00 AM Ken Javor <[email protected]> wrote: > This isn’t exactly what I was looking for, which was an algorithm, but it > is empirical data for particular GTEM cell, and it shows raw data in dBuV > and the predicted far field intensity at three meters. The 20 dB/decade > increase with increasing frequency is likely what prompted my analogy to t > e difference between antenna gain transmitting vs. antenna effective > aperture receiving. > > > > Ken Javor > Phone: (256) 650-5261 > > > ------------------------------ > *From: *"Heckrotte, Michael" <[email protected]> > *Reply-To: *"Heckrotte, Michael" <[email protected]> > *Date: *Tue, 13 Aug 2019 16:34:58 +0000 > *To: *<[email protected]> > *Conversation: *[PSES] GTEM cell used for RE measurement > *Subject: *Re: [PSES] GTEM cell used for RE measurement > > Hi Ken, > > TX and RX equations are reciprocal. The measurements themselves are > referenced to different locations, however we can address this. While the > conversion between antenna gain and antenna factor does involve wavelength > this will also be consistent between TX and RX after taking distance into > account. Free-space path loss also involves wavelength, and is also > reciprocal. > > Free-space theory, different distance references can be resolved using > simple equations assuming far-field path loss conditions: > > - For emissions we measure the field strength at [received by] the > antenna > - For immunity we want to know the field strength at some distance D > from the antenna > > > OATS theory: > > - Add in the ground plane and associated coupling > > > GTEM theory: > > - Associated coupling is different than OATS > - The measurement distance is not 3/10/etc. meters so need to develop > the equivalent of Friis equation for free-space path loss > > > Finally, in practice we don’t take the same coupling factors into account > when performing these two measurements: > > - For immunity we measure the field strength using a (small) > somewhat-ideal battery-powered isotropic probe with a fiber optic interface > thus there are no conducting cables. Next we put a (larger) product in > place of the probe and the coupling is different due to device size, > cables, etc. Then we claim that the product “sees” the previously measured > field strength but we don’t actually make a new field strength measurement > with the product and associated coupling mechanisms in place. Hence, we > artificially simplify immunity, though this isn’t necessarily bad – just as > our simulated test field will change due to device coupling, an actual > field in the real-world environment will also change, just not by the same > amount. > - For emissions the measurement is made with all the various device > coupling mechanisms in place. > > > Admittedly this doesn’t solve your task at hand, but perhaps it can shed > some light on where you might make some simplifying assumptions. > > > Best Regards, > Mike > > > *From:* Ken Javor <[email protected]> > *Sent:* Tuesday, August 13, 2019 7:27 AM > *To:* [email protected] > *Subject:* Re: [PSES] GTEM cell used for RE measurement > > I understand how the GTEM cell works for immunity. That is quite simple, > as you say. It’s not as simple for emissions. I think an analogy is the > equation for transmitting from an antenna is not reciprocal with receiving. > One is simply a function of gain, the other involves frequency/wavelength. > I too worked with GTEM cells in the past and understood better, and I too > did a search and found the very complex formulae (!) but now I need to be > able to get from a field intensity limit to the coupled potential at the > cell coax connector, and I’m not finding that, nor a way to get there. > > Ken Javor > Phone: (256) 650-5261 > > ------------------------------ > > *From: *"James Pawson (U3C)" <[email protected]> > *Reply-To: *"James Pawson (U3C)" <[email protected]> > *Date: *Tue, 13 Aug 2019 07:47:56 +0100 > *To: *<[email protected]> > *Subject: *Re: [PSES] GTEM cell used for RE measurement > > Hello Ken, > > Thinking aloud here: would such a factor exist as a generic formula? If I > think about this the other way round, putting 1V of RF into the coax > connector will generate 1V between the septum and ground (or between the > septums? septii? in the case of a differential GTEM like a Laplacell). > > This 1V will be spread over the distance between septum and ground, > dictating the overall field strength. This means cells with different > dimensions will have different factors. This is leaving aside the frequency > response / VSWR of the cell. > > I do recall seeing some formulae for correlation between GTEM and OATS/SAC > but that was a while ago. A quick google on the subject shows a few papers > with lots of big equations. > > I no longer have my old copy of EN 61000-4-20 but there might be some > interesting reference material in there. This standard also notes that many > of its provisions only apply to “small EUTs” which is anything below a > certain size in relation to the GTEM volume *or anything with cables > attached* (which are “under consideration”) > > > More practically, you could generate your own factors using an RF > generator and an isotropic field probe and using reciprocity to flip this > around to get a Volts out – field inside factor. > > > Additional: There’s also this NPL / York EMC guide on “The Use of GTEM > Cells for EMC Measurements” which has some interesting info in. Its been a > long while since I’ve read it in any depth though. > https://www.npl.co.uk/special-pages/guides/gpg65_gtem > > > Hope some of this is useful. > All the best > James > > > > James Pawson > EMC Problem Solver > > > * Unit 3 Compliance *Design for EMC / Pre Compliance / Problem Solving / > EMC Testing / Consultancy / Environmental & Vibration > www.unit3compliance.co.uk <http://www.unit3compliance.co.uk> > <http://www.unit3compliance.co.uk> <http://www.unit3compliance.co.uk/> > <http://www.unit3compliance.co.uk/> -- 07811 139957 > Opening Hours: Tuesday to Friday, 0830 to 1800. Closed Monday. > > > *From:* Ken Javor <[email protected]> > *Sent:* 12 August 2019 15:44 > *To:* [email protected] > *Subject:* [PSES] GTEM cell used for RE measurement > > Does anyone out there have a simple transducer factor that yields rf > potential at the GTEM cell vertex coaxial connector for a given vertical > electric field component coupling to the cell septum? Purpose is to > calculate gain/noise figure specs for a preamplifier used between spectrum > analyzer and GTEM cell to measure RE. > > Thank you. > > Ken Javor > Phone: (256) 650-5261 > > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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